窄流道内压力对汽泡动力学特性的影响

本文采用流道间隙为2 mm和2.5 mm的实验段,以水为工质,对流动过冷沸腾时不同压力下汽泡动力学特性进行了研究.发现在较高压力下,壁面要达到更高的过热度才会出现汽泡,汽泡生长时间更长,汽泡直径较小,生长速度较低;较高压力下生长的汽泡成串排列在加热壁面上,其沿壁面滑移距离随着过冷度和热流密度的增加而增大.较高压力下小汽泡容易发生滑移现象的原因在于高压力使气泡的直径减小,导致气泡的受力发生改变,而使气泡的行为发生改变.     

液体过冷度对沸腾换热中汽泡合并及传热的影响

本文对三种过冷度下,微尺度加热片阵列上的沸腾换热现象做了实验研究,实验采用两组核化点,每个核化点包含12个大小约为0.1 mm×0.1 mm的微型加热片。利用高速摄像技术从汽泡的底部对汽泡生长及合并现象进行观测,并用高速数据采集系统同步记录不同加热片热流密度.实验探究了不同过冷度下的汽泡合并现象,以及对应的汽泡直径变化规律、汽泡脱离频率和热流密度曲线。文章分析了不同的合并现象并探究了过冷度对沸腾传热的影响。     

狭窄加热表面上核态沸腾中汽泡合并特性的实验研究

狭窄表面的沸腾可以减少沸腾汽泡在观测方向上的干扰,有利于更清晰地研究汽泡生长和合并特性。为探究沸腾过程中汽泡的合并,本文基于常规机械加工的狭窄加热表面,通过高速摄像机完整记录了加热表面上汽泡的生长,合并和脱离过程,并分析了汽化核心间距,汽泡脱离直径对汽泡合并的影响。通过对近壁面处沸腾动态图像的观察,观测到了汽泡合并的不同特性。当沸腾过程中热流密度较低时,近壁面处只发生汽泡水平和倾斜合并,并常常同时存在。而竖直方向上的汽泡合并仅发生在热流密度相对较高时,且常伴随着水平和倾斜方向的合并。相邻汽泡间的合并现象常通过汽泡中心距与合并汽泡脱离直径间的关系来衡量。本实验结果表明,相邻汽化核心上的汽泡发生合并时,汽泡中心距与汽泡脱离直径满足S/D1.5。同时,相较于粗糙表面,光滑表面的S/D变化范围较小,且平均值有减小趋势。这一结果有助于进一步研究表面结构对汽泡合并的影响。     

含内热源球床通道内过冷流动沸腾汽泡行为特性研究

为加深对含内热源球床通道内流动过冷沸腾换热机理的研究,采用高速摄像仪对通道内流动过冷沸腾时的汽泡行为进行了可视化实验研究。结果表明:汽泡在通道内很容易受到球体的阻碍而附着于球面上,受到阻碍的汽泡将沿球面进行滑移运动;汽泡存在大量的"重生现象",即生成的汽泡在生存阶段可能出现多次生长的情况;在球与球接触的角区,会产生稳定的汽化核心点。角区的结构形式对汽泡的脱离直径及脱离频率产生较大的影响。     

核态沸腾中汽泡动力学及传热机理分析

汽泡的传热及生长特性研究对于揭示核态沸腾的机理具有重要作用。本文介绍了利用高速摄像技术对核态沸腾中汽泡生长及运动现象进行实验观测,并用高速数据采集系统记录汽泡一个生长周期不同阶段的热流密度的方法。在不同的过冷度及壁面温度条件下,观测了汽泡的生长、周期性滑移、颈化和脱离现象。计算并绘制出不同条件下一个汽泡生长周期内的热流密度曲线,与汽泡图片相对应,分析并讨论了造成这些现象的机理。     

竖直矩形窄缝通道过冷流动沸腾的实验研究

本文以去离子水为实验工质,对常压下竖直窄缝通道内过冷流动沸腾的气泡特性进行了实验研究。通过可视化实验分析发现,汽化核心密度主要受壁面过热度影响,气泡脱离直径受壁面过热度、主流过冷度以及质量流速影响。拟合了汽化核心密度和气泡脱离直径的经验关系式,预测结果与实验值误差较小。     

垂直窄缝流道内过冷沸腾时的汽泡行为

通过高速摄像可视化研究发现,在p=1.3～2MPa时,F-12质初始汽泡在壁面以小于0.1m/s的低速滑动中生长。热流密度和断面平均过冷度等参数对初始汽泡影响较大,热流密度越高,沸腾越早发生;小汽泡(d=0.01～0.07mm)运动速度在0.1～0.2 m/s左右,而较大(d=0.1～0.3 mm)汽泡的运动速度在0.25～0.7m/s左右。较大汽泡聚合小汽泡的过程是汽泡从小汽泡生长为大汽泡乃至于汽层的主要形式。     

偏滤器微纳表面冷却通道的过冷流动沸腾数值模拟

为模拟偏滤器水冷模块微纳米结构化表面的传热特性,结合微纳表面可视化微观观察实验数据,在现有气泡参数模型的基础上,对接触角、气泡脱离直径、气泡脱离频率、汽化核心密度等参数模型进行修改,提出可模拟微纳表面过冷流动沸腾传热效果的计算模型。用该模型对压力为4MPa、速度为10m·s-1、进口温度为423K的偏滤器水冷结构中的过冷流动沸腾进行计算,得到常规水冷通道与微纳表面水冷通道各结构的温度与气相体积分布。计算结果表明,微纳表面的平均传热系数提高约一倍;在无氧铜与铬锆铜的许用温度范围内,微纳表面通道偏滤器承受的稳态热流密度可达14MW·m-2。     

开孔金属泡沫池内沸腾换热特性研究

常压下用去离子水作为工质,在沸腾池底部单独或两两叠加地放置3种PPI的泡沫金属进行池内沸腾实验研究;分析了不同过热度下的热流密度和不同热流密度下池沸腾的传热系数;对沸腾汽泡在单层或双层泡沫金属表面产生、融合和脱离进行了可视化的研究。实验发现:不同PPI泡沫金属的叠加方式,会影响沸腾传热面积和产生汽泡的核心数,以及汽泡脱离受热壁面的动态特性。本实验为多孔介质池沸腾传热特性研究提供了实验指导和基础数据。     

基于汽泡附壁接触直径的窄流道汽泡脱离预测模型

汽泡脱离是沸腾系统中非常重要的现象,对沸腾流动和传热都有重要的影响。汽泡脱离过程是非常复杂的,受到系统工况参数以及物性参数的综合影响;但汽泡在生长过程中的受力状况是决定汽泡发生脱离的根本因素。对于窄流道内的汽泡,由于受到流道壁面的限制,汽泡与加热壁面间将存在较大的接触面。本文基于汽泡生长过程中的附壁接触直径,详细分析作用在汽泡上的各个受力,并建立各个受力的计算关系式,从而得到汽泡脱离的预测模型。模型采用本研究小组在竖直窄流道内得到的实验数据进行了验证,预测结果与实验研究吻合很好。     

Effect of Bubble Contact Diameter on Bubble Departure Size in Flow Boiling

The growth and departure of bubbles on a vertical heated wall of an annular channel in flow boiling were observed by a high-speed camera, and the bubble contact diameter and bubble departure diameter were measured. It was found that bubble departure diameters were different among different nucleation sites in the same boiling area (same test condition, same surface roughness). Force balance on a single bubble attached to the heated surface was also analyzed to explain this phenomenon. The theoretical research results show that bubble contact diameter has a significant influence on bubble departure diameter, and the corresponding departure diameter is larger as the bubble contact diameter is larger. This agrees with the visual experimental results.     

DMFC阳极CO_2气泡生长及脱离特性研究

通过对竖直放置直接甲醇燃料电池水平流道内扩散层壁面上CO_2气泡的受力分析,建立描述气泡生长动力学方程,获得CO_2气泡生长速率和气泡脱离直径的计算方法。计算结果表明:CO_2气泡生长和脱离主要受浮力、曳力、剪切升力和表面张力的控制;气泡生长速率随电流密度和接触环直径的增大而增大;甲醇溶液流速增加,气泡脱离直径变小,且流速对气泡脱离直径的影响随接触环直径减小而变大;电池放电电流密度的变化对气泡脱离直径几乎没有影响;温度和甲醇浓度增加,均使气泡脱离直径略有减小;扩散层表面润湿性越好,气泡的脱离直径越小。     

DNS of turbulent flow with hemispherical wall roughness

The present study of the effect of roughness density on the mean flow turbulence parameters is motivated by the need for new generation of boundary conditions for multiphase computational multiphase fluid dynamics (CMFD) models applied to boiling flows. Effect of roughness element density on the turbulent flow in a channel is quantified through direct numerical simulations (DNSs). The Navier--Stokes equations are solved using finite element method and bubbles are approximated as rigid near-hemispherical obstacles at the wall. Six different cases were analysed including channel flow with smooth wall and channel flow with rough wall for five different bubble nucleation site densities. Friction factor and the law of the wall was calculated and compared with the previously published results. Existing correlations for nucleating bubble site density dependency on a wall heat flux were used to obtain a relation between the heat flux and the friction factor, leading to the law of the wall dependency on the heat flux. This separate effect study provides new guidelines on how the heat flux in subcooled boiling regime affects the turbulence behaviour near the wall and guides the computational fluid dynamics model development for boiling two-phase flows.     

Experimental study of bubble departure characteristics in forced convective subcooled nucleate boiling

Visualization experiments in upward forced convective nucleate boiling flows were carried out. The bubble growth and collapse have been measured using high-speed photography technique with distilled water under atmospheric pressure. The experiments show that the bubbles depart from nucleating sites shortly after nucleation and slide along the heater surface. The bubbles grow while sliding, attain a maximum size, then lift from the surface sometime during condensation, and quickly vanish in the bulk liquid. Parametric studies show that bubble diameter and departure frequency increases with an increase in heat flux, a decrease in subcooling, and a decrease in mass flux.     

环形小通道内过冷沸腾的气泡行为研究

本文对去离子水在竖直环形小通道内流动过冷沸腾时的气泡行为进行了可视化研究,观察到了气泡在流动沸腾过程中的生长,合并,脱离,滑移及气泡脱离壁面后的运动等现象,并结合气泡动力学理论模型及气泡滑移模型,对实验结果进行了对比分析,总结了本实验通道中气泡行为的特性。     

水平管内环状流动核态沸腾换热模型

本文在已有的池和外掠平板核态沸腾机理研究的基础上,提出了系统的水平管内饱和流 动核态沸腾理论,建立了水平管内环状流下流动核态沸腾换热的组合模型,并对该模型进行了简化 和数值求解。最后,将计算结果与ＨＦＣ－１３４ａ实验数据进行了比较;结果表明二者一致性较好。     

Experimental Investigation of Subcooled Vertical Upward Flow Boiling in a Narrow Rectangular Channel

Accurate models for the onset of nucleate boiling, density of active nucleation sites (N_a), bubble departure size (D_d), and departure frequency (f_d) are essential to the success of computational fluid dynamics analysis of two-phase thermal-hydraulics involving subcooled flow boiling in nuclear reactor systems. This work presents an experimental study of subcooled flow boiling in a vertical upward narrow rectangular channel that mimics the flow passage in the plate fuel assembly of boiling water reactors. The experiments are conducted over a range of mass flux (G = 122–657 kg/m²s), inlet subcooling (ΔT_sub = 4.7–33.3?C), and heat flux (q″ = 1.7–28.9 W/cm²). Based on the experimental data, empirical correlations are developed for the prediction of onset of nucleate boiling, N_a, D_d, and f_d for given flow conditions. These correlations are valid in the nucleate boiling regime when the wall superheat is less than 12°C and can be incorporated in the computational fluid dynamics codes to enable more precise simulation of subcooled flow boiling heat transfer and two-phase flow in nuclear energy applications.     

纳米尺度下气泡核化生长的分子动力学研究

采用分子动力学方法模拟纳米尺度下液体在固体壁面上发生核化沸腾的过程,主要研究壁面浸润性对气泡初始核化过程和气泡生长速率的影响以及固-液界面效应在液体核化沸腾的能量传递过程中所起到的作用.研究结果发现：壁面浸润性越强,气泡在固壁处越容易核化.该结果与经典核化理论中“疏水壁面易于产生气泡”的现象产生了明显的区别.其根本原因是在纳米尺度下,固-液界面热阻效应不能被忽略.一方面,在相同的壁温下,通过增强固-液相互作用,可以显著降低界面热阻,使得热量传递效率提高,导致靠近壁面处的流体温度升高,气泡核化等待时间缩短,有利于液体沸腾核化.另一方面,气泡的生长速率随着壁面浸润性的增强而明显升高.当气泡体积生长到一定程度时,会在壁面处形成气膜,从而导致壁面传热性能恶化.因此,通过壁面的热流密度呈现出先增大后减小的规律.     

VISUALIZATION OF NUCLEATE FLOW BOILING FOR AN R22/R114 MIXTURE AND ITS COMPONENTS

Visualization of bubble nucleation during forced-convective flow inside a horizontal, electrically heated quartz tube was done in order to establish a comparison of this phenomenon between refrigerant mixtures and their pure components. The specific phenomena investigated were the suppression of nucleation due to increased mass flow quality while holding all other conditions fixed, and the comparison of the nucleate activity of the binary mixture to the nucleate activity of the pure components. The fluids investigated were a 37.7 mol° R22/62.3 mol° R114 binary mixture and the individual components R22 and R114. These fluids were pumped through an abraded, electrically heated quartz tube. A 16-mm high-speed camera was used, at 7000 frames/s, to film the boiling process. Detailed measurements of bubble frequency and bubble size were possible at low pressures, allowing direct calculation of the latent heat load required to nucleate a single bubble. Further work is required to develop a method that ensures statistically sound bubble frequency measurements. However, the standard deviations of the bubble diameter measurements were acceptable. The films were used to visually demonstrate the suppression of nucleation with increase in quality for R114, R22, and an R22/R114 mixture. The films suggest that, for a given quality, RI14 exhibits much more nucleation than either R22 or the mixture, while the amount of nucleation demonstrated by R22 and the mixture was comparable even though the mixture was mostly R114 by mole. Arguments using the latent heat of vaporization, the vapor density, and the liquid thermal conductivity have been made to explain the visual trends.